But we don't have to use it twice:
كلما ازداد عدد الأولاد قلت حرية الأبوين
كلما طالت القصة صارت مملة

I'd like to add what I found in my grammar book:
The sentence with "kullama" is أسلوب شرط but kullama doesn't put the verb in the case of الجزم , like لو، لولا، إذا، لمّا . And كلما: ظرف للماضي، ولا يليه إلا الفعل الماضي

My preliminary findings have it that repeating كلما does happen quite often, but that it is a common mistake. If you conduct a google search for تكرار كلما you'll find the topic of "repeating كلما " being discussed. In the end, it might come down to "descriptivism vs. prescriptivism" which is altogether another issue but relevant to our question here.

My preliminary findings have it that repeating كلما does happen quite often, but that it is a common mistake. If you conduct a google search for تكرار كلما you'll find the topic of "repeating كلما " being discussed. In the end, it might come down to "descriptivism vs. prescriptivism" which is altogether another issue but relevant to our question here.

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It would make sense if it was colloquial influence in writing to double كلما - I don't know what the classical grammarians have to say about it. If you use كلما in colloquial without repetition, and with the non-past verb, I believe it can mean "whenever" rather than "the more... the more...". كلما بتيجي لبيتي بتطلب مني مصاري : whenever you come to my house you ask me for money. Thus doubling كلما in MSA writing may be an attempt at avoiding this secondary meaning.

Apparently, it's not just doubled in the context of "the more ...the more". I was thinking that it might a little like the English: "Everytime I tell you not to do it, everytime you insist on doing it", as if for emphasis. This is especially this case, I think, when the first clause is somewhat long in English.

Moreover, I think that the comparative "more" is achieved through using an equivalent comparative in Arabic like أكثر or أطول , etc. depending on the context. The repetition of كلما could be, I think, due to it being repeated in English, making it a case of linguistic borrowing.

Hello
I must have used the construction with
بقدر ما
in the wrong context or in an improper way in my previous posting, but here are examples taken from the internet for you to comment on. In my opinion they express the same meaning as
كلما

Although بحسبما seems to be used mainly with the meaning of "according to what" as in
بحسبما قال\أفاد\أذكر\ورد
I have come across it with a correlative meaning similar to كلما
ولكن بحسبما اذلّوهم هكذا نموا وامتدّوا

Although بحسبما seems to be used mainly with the meaning of "according to what" as in
بحسبما قال\أفاد\أذكر\ورد
I have come across it with a correlative meaning similar to كلما
ولكن بحسبما اذلّوهم هكذا نموا وامتدّوا

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Yes, here it means "as much as they have..." or "to the degree that they have..."